1. Field of the Invention
The present invention relates to position detection devices for detecting a position of a sheet, conveyance devices, and image formation devices.
2. Description of the Related Art
Image formation devices control recording sheet conveyance and image formation so that an image is formed at a predetermined location on a recording sheet. If a recording sheet being conveyed is skewed relative to the conveyance direction (skew) or misaligned in a direction (lateral direction) perpendicular to the conveyance direction (lateral misalignment), an image is not formed at a predetermined location.
US 2009/0295079 proposes a skew correction mechanism that corrects a skew of a recording sheet. Japanese Patent Laid-Open No. 05-124752 proposes a correction mechanism (lateral registration correction mechanism) that corrects a lateral misalignment. In particular, Japanese Patent Laid-Open No. 05-124752 proposes a technique of detecting the position of a lateral edge of a recording sheet using a line sensor provided in the conveyance path, and shifting a roller in the lateral direction so that the position of the lateral edge is aligned with a predetermined position. Here, the lateral edge refers to one of the four sides of a rectangular recording sheet that is parallel to the conveyance direction, i.e., an edge portion in the lateral direction of the sheet.
To accurately align the lateral edge with a predetermined position, it is necessary to accurately detect the position of the lateral edge. Japanese Patent Laid-Open No. 2004-25579 describes a technique of detecting a change of the level of an output signal from a comparator from an output level caused by reflected light from a surface of a recording sheet to an output level caused by reflected light from the black surface of the conveyance guide and thereby detecting the position of the lateral edge.
Japanese Patent Laid-Open No. 2004-25579 does not describe how the threshold of the comparator is determined. The present inventor considers that the threshold should be set to a middle level between the output level (light level) caused by reflected light from a surface of a recording sheet and the output level (dark level) caused by reflected light from the black surface of the conveyance guide. This is because of several reasons. A recording sheet may have an undulated surface during conveyance. As a result, the recording sheet surface may deviate from the position of the focal point of the line sensor for detecting the lateral edge position. In this case, the change from the light level to the dark level is not steep, but rather moderate. Thus, a signal having an intermediate level between the light and dark levels is output over a relatively long interval, and therefore, the lateral edge position is likely to be incorrectly detected. Therefore, it is necessary to set the comparator threshold to the exact middle level between the light and dark levels.
On the other hand, the reflection or transmission characteristics (sheet type) of the recording sheet surface often vary among brands of the recording sheet. For example, plain paper and recycled paper have significantly different optical densities of the surface. In other words, the light level varies depending on the sheet type. Therefore, it is necessary to set the threshold, depending on the sheet type or the light level actually measured.
Note that when the light level is measured, it is necessary to take into consideration variations in the sensitivity of a plurality of light receiving elements included in the line sensor. In other words, the plurality of light receiving elements do not necessarily output the same level of a signal in response to the same amount of incident light. For example, it is assumed that while a light receiving element having a high sensitivity is actually used to detect the lateral edge position, the threshold is determined with reference to the light and dark levels of a light receiving element having a low sensitivity. The light level of the high-sensitive light receiving element is higher than that of the low-sensitive light receiving element. Therefore, the threshold is set to be lower than the middle value between the light and dark levels of the high-sensitive light receiving element, so that the accuracy of detection of the lateral edge position decreases. Therefore, it is necessary to determine the threshold based on the light and dark levels of a light receiving element that actually detects the lateral edge position.